In recent years, secondary batteries such as non-aqueous electrolyte secondary batteries or nickel-metal hydride batteries have become important as the power source mounted in automobiles using electricity as the driving source, or as the power source mounted in personal computers or mobile terminal and other electrical products. Particularly, it is expected that non-aqueous electrolyte secondary batteries that are lightweight and have a high energy density will be suitably used as a high output power source for use in automobiles.
In the construction of a typical lithium ion secondary battery as a non-aqueous electrolyte secondary battery, there is provided an electrode active material layers on the surface of an electrode collector that capable of reversible occlusion and discharge of lithium ions, and more specifically, there is provided a cathode active material layer and anode active material layer. For example, in the case of the cathode, cathode active material that is composed of a composite oxide that includes transition metals such as lithium, nickel and the like as metal elements are dispersed in to a suitable solvent that is composed of an aqueous solvent such as water or various kinds of organic solvent to obtain a paste-like composition or slurry-like composition (hereafter, these composition will simply be referred to as “paste”), and a cathode active material layer is formed by applying that paste to an electrode collector.
Incidentally, of the composite oxides of a cathode active material of a lithium ion secondary battery, a so-called lithium nickel composite oxide constructed with nickel as the main material: LiNi1-xMxO2 (M is one kind or two or more kinds of metal elements other than nickel) has advantages over conventional lithium cobalt composite oxides in that it theoretically has greater lithium ion occlusion capacity, and it is possible to reduce the amount of costly metal material such cobalt that is used, so is gaining much attention as a suitable cathode material for the construction of a lithium ion secondary battery.
When using a lithium nickel composite oxide, which is obtained by a conventionally proposed manufacturing method, as a cathode active material, there is a problem in that even though the charge capacity and discharge capacity is higher than that of a lithium cobalt oxide, the cyclability is inferior. Moreover, when used in high-temperature environments or low-temperature environments, lithium nickel composite oxide has a disadvantage in that it is comparatively easy for the battery performance to become impaired.
In order to improve cyclability, adding or substituting different kinds of elements into the lithium nickel composite oxide is being tried. For example, JP 8-78006 (A) discloses a cathode active material that is composed of a composite oxide having layered structure and expressed by the general expression: LiaNibM1cM2O2, where M1 is Co, and M2 is one or more kind of element that is selected from among at least B, Al, In and Sn.
With the cathode active material of this disclosure, the cyclability is improved, however, depending on the existence of added elements, the capable range of intercalation and deintercalation of lithium ions of the cathode active material becomes narrow, so there is a tendency for the discharge capacity to decrease. This decrease in discharge capacity is known to become particularly remarkable in heavy load conditions where the discharge current is large, or in low-temperature efficient discharge conditions where the mobility of electrolytes becomes small at low temperature.
The output characteristics at high temperature or low temperature of secondary battery are extremely important characteristics when the battery is used in equipment that is used in environments where there is a large change in temperature, and particularly when considering use an cold regions, it is necessary for the battery to have sufficient output characteristics at low temperature.
In an attempt to improve the output characteristics at low temperature, JP 11-288716 (A) discloses a cathode active material that is composed of lithium nickel cobalt oxide formed of spherical or elliptical secondary particles having an average particle size of 5 μm to 20 μm and in which primary particles are collected in a radial fashion, with this cathode active material being expressed by the general expression: LixNiyCo1-yO2 (where 0<x<1.10, 0.75<y<0.90).
With the cathode active material of this disclosure, uniform intercalation and deintercalation from the surface of the secondary particles to inside the crystal is possible, and a lithium ion secondary battery having high capacity, excellent heavy load characteristics and excellent low-temperature efficient discharge characteristics can be obtained. However, when the cathode active material described above is used, the surface of the secondary particles is covered by conductive materials, binding agent, or gas that is adsorbed into the surface the secondary particle during the formation of the cathode active material, so mobility of the lithium ions is obstructed, it is feasible that particularly low-temperature efficient discharge characteristics will not be sufficiently obtained.
On the other hand, in attempts to improve the large current charge and discharge characteristics, or in other words, improve the output characteristics, attention has been placed on the size of the primary particles and secondary particles of the cathode active material. For example, JP 2000-243394 (A) discloses that by keeping the ratio “D50/r” of the average length “r” in the short length direction of the primary particles, and the particle size “D” when the volume cumulative frequency of particle size distribution of secondary particles reaches 50% within a specified range, cathode active material having high discharge potential, excellent large current characteristics and good cyclability can be obtained.
The crystallinity of the cathode active material is also described, where preferably the relationship between the half width (full width at half maximum) FWHM (003) and FWHM (104) of the diffraction peaks of the (003) surface and (104) surface of the Miller indices hkl of X-ray diffraction of composite oxide that can be used as a cathode active material is 0.7≦FWHM (003)/FWHM (104)≦0.9, and furthermore, preferably, 0.1°≦FWHM (003)≦0.16° and 0.13≦FWHM (104)≦0.2°.
These indicate the effect of the crystallinity of the cathode active material on the output characteristics, however, they are related to the relationship between the large charge and discharge characteristics and the relative orientation of a plurality of crystal surfaces, and there is no mention of improvement of the low-temperature output.
Moreover, JP 10308218 (A) discloses a cathode active material for a lithium ion secondary battery that is expressed by the general expression: LiMO2 (where M is at least one element selected from among the group of Co, Ni, Fe, Mn and Cr), and is composed of particles that are a collection of single crystals with minute crystallites as the unit, where the shapes of the crystallites and the particles are sterically nearly isotropic in shape, and when expressed in term of crystallites, is within the range of 500 {acute over (Å)} (Angstroms) to 750 {acute over (Å)} in the (003) vector direction and 450 {acute over (Å)} to 1000 {acute over (Å)} in the (110) vector direction.
In this disclosure, the size of the crystallites is used for expressing the sterical isotropic shape of the particles, however, there is no mention of the effect of the size of the crystallites themselves. Moreover, the object is to achieve both thermal stability during charging and good charge and discharge cyclability, and is not related to an improvement in low-temperature output.
On the other hand, attempts are being made to improve the cathode active material by placing attention on the nickel composite compound used as the raw material for the lithium nickel composite oxide, or in other words the characteristics of the precursor of the cathode active material. As the method for manufacturing the lithium nickel composite oxide, a typical method of mixing and calcining a lithium compound and a nickel composite compound that is composed of nickel, cobalt and metal elements M is used. Hydroxides, oxides, nitrates and the like are used in the nickel composite compound, however, because it is easy to control the shape, particle size and crystallinity of the materials, typically a hydroxide or an oxide that is obtained by calcining the hydroxide is used.
For example, JP 7-335220 (A) discloses manufacturing a cathode active material composed of lithium nickel oxide that is expressed by the general expression LiNiO2, wherein the lithium nickel oxide is obtained by performing heat treatment in an oxidizing atmosphere of nickel hydroxide and lithium hydroxide that are formed into secondary particles that are a collection of primary particles having a particle size of 1 μm or less.
Furthermore, by using a particle structure in which the opening section of the primary particles having a layered structure of nickel hydroxide is oriented toward the outside of the secondary particles, the end surface of the generated LiNiO2 layer also maintains that shape and is oriented toward the outside of the powdered particles, so intercalation and de-intercalation of Li during charging and discharging can advance smoothly.
However, in this disclosure, only the particle shape of the cathode active material that is obtained and maintaining the orientation is disclosed, and there is no mention of the effect of nickel hydroxides on the crystallinity of the cathode active material that is obtained.
Moreover, JP 11-60243 (A6) discloses a nickel hydroxide as a precursor to a cathode active material that is expressed by the general expression: Ni1-yAx(OH)2 (where A is cobalt or manganese, 0.10<x<0.5), and is composed of a layered body having uniform crystal orientation or a single crystal, with particle size of primary particles being 0.5 to 5 μm, and the full width at half maximum found through X-ray diffraction obtained by taking a sampling with the easiest orientation is (001)<0.3 deg., (101)<0.43 deg., and the peak intensity ratio is I (101)/I (001)<0.5.
In the case of this disclosure, the thermal characteristics during charging are improved by achieving the accretion of primary particles in the raw material stage instead of by sintering during calcination, and without a decrease in battery characteristics of the lithium ion secondary battery. However, nothing is mentioned about the crystallinity of the obtained cathode active material, or about the effect crystallinity of nickel hydroxide as a raw material, and does not mention anything about improving the low-temperature output.
Furthermore, the relationship between the battery characteristics of a cathode active material and the power characteristics of the material is being studied. For example, the applicants of this disclosure proposed in JP 2000-30693 (A) a hexagonal lithium nickel composite oxide having a layered structure that is expressed as [Li]3n[Ni1-x-yCoxAly]3b[O2]6c (where the subscripts to the brackets [ ] represent sites, and x and y satisfy the conditions 0<×≦0.20, and 0<y≦0.15), and with the objective of reducing irreversible capacity, the structure is such that secondary particles are formed by collecting a plurality of primary particles of the lithium nickel composite oxide, with the average particle size of the primary particles being 0.1 μm or greater. Moreover, it is disclosed that there is a linear correlation between the average particle size of the primary particles and the crystallite diameter that is calculated from the half width of the 003 peak in the X-ray diffraction pattern, with the crystallite diameter that is calculated from the half width of the 003 peak in the X-ray diffraction pattern being 40 nm (400 {acute over (Å)}) or greater, and more specifically, within the range of 430 {acute over (Å)} to 1190 {acute over (Å)}.
However, in that disclosure, regulating the powder characteristics of the battery characteristics because of its relationship to the reduction in irreversible capacity of the battery is disclosed, however, the relationship between the low-temperature output and the powder characteristic is not studied, and nothing is disclosed for improving the low-temperature output.